The competitive and synergistic interactions of Drechslera sorokiniana and Curvularia geniculata on leaf spot development on Poa pratensis

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Abstract
Research was initiated to determine interactions between D. sorokiniana and C. geniculata in a potential disease complex on P. pratensis. Inoculations with D. sorokiniana resulted in the greatest disease severity at 20 and 25.degree. C. Disease severity was minimal with C. geniculata inoculations at all temperatures. Inoculations with the combination of both organisms displayed competitive and synergistic interactions that were temperature dependent. Combination inoculations reduced disease severity below that produced by D. sorokiniana alone and increased disease severity above that produced by C. geniculata alone at 20 and 25.degree. C and was indicative of a competitive interaction. Inoculations with the combination of the organisms increased disease severity above that produced by either organism alone at 30.degree. C, indicative of a synergistic effect at higher temperatures. Reisolation of the organisms from lesions produced from inoculations with the combination of the organisms indicated that C. geniculata competes more successfully for infected tissue than D. sorokiniana but is dependent on lesions initiated by D. sorokiniana to establish successfully. Competition for tissue colonization by C. geniculata is greatest at temperatures of 25 to 35.degree. C. C. geniculata should be recognized as a weak, high-temperature, primary-leaf-infecting pathogen of P. pratensis. A true disease complex relative to disease severity can exist between the organisms at 30.degree. C that is dependent on lesions initiated by D. sorokiniana and reflects the high-temperature aggressiveness of C. geniculata. Tissue invaded by both organisms is more successfully colonized by C. geniculata. Tissue invaded by both organisms is more successfully colonized by C. geniculata at higher temperatures. The necrotic portions of lesions produced by D. sorokiniana, and on which this organism does not sporulate well, is the probable site of establishment for C. geniculata where the organisms are found together.